A wireless local area network (WLAN) is widely used to provide access to the internet with mobile devices. To improve the throughput in a WLAN, the IEEE 802.11n standard adopts a multiple input multiple output (MIMO) system that transmits a plurality of streams with multiple antennas and, at the same time, receives the streams with multiple antennas. However, the IEEE 802.11n standard is still based on a point-to-point transmission scheme. When there are more stations connected to the access point (AP), the AP has to hold the transmission for some stations while transmitting to another and wait for an empty time slot.
In order to transmit to multiple stations (STA) on the same channel (frequency band) in a multi-user (MU) system, the downlink (DL) channels may be orthogonalized to create virtual spatial channels. Some conventional systems are described in “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multi-User MIMO Channels”, IEEE Transactions On Signal Processing, Vol. 52, No. 2, February 2004, Q. Spencer, A. L. Swindlehurst, and M Haardt. In this publication, and in later references, a set of pre-coding matrices is computed at the AP for a group of downstream users. The effective pre-coded channel is block diagonalized so that the interference between stations is minimized.
For this system to work effectively there must be cooperation between the AP and the remote stations. First, the stations must feedback channel information to the AP for pre-coder computation. In certain cases where the individual station channels are highly correlated, pairs of stations may not be compatible to be grouped for MU transmission due to a poor solution space for MU pre-coding matrices. In addition, MU channels are more sensitive to changing channel conditions, since variations in the channel can lead to loss of pre-coded orthogonality and quickly lead to high interference conditions. Many designs only consider grouping without regard to compatibility of the stations, while other systems do not provide real time metrics to assess the status of the channel condition. These two factors can limit the performance of a MU-MIMO downlink (DL). Hence, there's a need to provide a method to form stations into compatible multi-user (MU) groups, and the need for a mechanism to quickly identify interference conditions caused by variations in the wireless MU channel.
Therefore, a multi-user (MU), or multi-station, transmission system with MIMO system can be provided. The AP in a MU-MIMO system can transmit data to multiple stations either with multiple antennas or a single antenna at the same time, such that the AP can serve more stations at the same time. However, there's still a need for a method to improve the reliability and functionality of a MU system.